GaN group semiconductor light-emitting element with concave and convex structures on the substrate and a production method thereof

ABSTRACT

Concaves and convexes  1   a  are formed by processing the surface layer of a first layer  1 , and second layer  2  having a different refractive index from the first layer is grown while burying the concaves and convexes (or first crystal  10  is grown as concaves and convexes on crystal layer S to be the base of the growth, and second crystal  20  is grown, which has a different refractive index from the first crystal). After forming these concavo-convex refractive index interfaces  1   a  ( 10   a ), an element structure, wherein semiconductor crystal layers containing a light-emitting layer A are laminated, is formed. As a result, the light in the lateral direction, which is generated in the light-emitting layer changes its direction by an influence of the concavo-convex refractive index interface and heads toward the outside. Particularly, when an ultraviolet light is to be emitted using InGaN as a material of a light-emitting layer, a quantum well structure is employed and all the layers between the quantum well structure and the low temperature buffer layer are formed of a GaN crystal, removing AlGaN. The quantum well structure preferably consists of a well layer made of InGaN and a barrier layer made of GaN, and the thickness of the barrier layer is preferably 6 nm–30 nm.

TECHNICAL FIELD

The present invention relates to a semiconductor light-emitting element(hereinafter to be also referred to simply as a light-emitting element).Particularly, this invention relates to a semiconductor light-emittingelement having a light-emitting layer made of a GaN group semiconductorcrystal (GaN group crystal).

BACKGROUND ART

The basic element structure of a light-emitting diode (LED) comprises acrystal substrate, and an n-type semiconductor layer, a light-emittinglayer (including DH structure, MQW structure, SQW structure) and ap-type semiconductor layer sequentially grown thereon, wherein each ofthe n-type layer, the conductive crystal substrate (SiC substrate, GaNsubstrate and the like) and the p-type layer has an external outletelectrode.

For example, FIG. 8 shows one exemplary constitution of an element (GaNgroup LED) comprising a GaN group semiconductor as a material of alight-emitting layer, wherein a GaN group crystal layer (n-type GaNcontact layer (also a clad layer) 102, a GaN group semiconductorlight-emitting layer 103 and a p-type GaN contact layer (also a cladlayer) 104) are sequentially laminated on a crystal substrate 101 bycrystal growth, and a lower electrode (generally an n-type electrode)105 and an upper electrode (generally a p-type electrode) 106 are set.In this specification, the layers are mounted on a crystal substrate(downside) and the light goes out upward, for the explanation's sake.

In LED, an important issue is how efficiently the light produced in thelight-emitting layer can be externally taken out (what is called,light-extraction efficiency). Therefore, various designs have beenconventionally tried such as an embodiment wherein an upper electrode106 in FIG. 8 is rendered a transparent electrode so that the lightheading upward from the light-emitting layer will not form an obstacleto the outside, an embodiment wherein the light heading downward fromthe light-emitting layer is returned upward by forming a reflectivelayer and the like.

For the light emitted from the light-emitting layer in the verticaldirection, the light-extraction efficiency can be improved by making theelectrode transparent or forming a reflective layer, as mentioned above.However, of the lights advancing in the spreading direction of thelight-emitting layer (direction shown with a thick arrow in alight-emitting layer 103 in FIG. 8, hereinafter to be also referred toas a “lateral direction”), the light that reaches the sidewall withinthe total reflection angle defined by a refractive index differentialcan be externally emitted, but many other lights repeat reflectionbetween, for example, sidewalls, are absorbed in an element,particularly by a light-emitting layer itself, attenuated and disappear.Such lights in the lateral direction are enclosed by upper and lowerclad layers or a substrate (sapphire substrate) and an upper clad layer,or a substrate and an upper electrode (further, a coating substance onthe outside of the element and the like), and propagated in the lateraldirection. The light that propagates in the lateral direction occupies alarge portion of the entire light amount produced by a light-emittinglayer, and in some cases it amounts to 60% of the whole.

With regard to a flip-chip type LED (light goes out through a substrate)to be mounted with the substrate on the upper side, an embodiment isknown wherein a side wall of a laminate, which is an element structure,has an angle and the side wall is used as a reflection surface towardthe substrate side, so that such light in the lateral direction can bereflected in the substrate direction. However, cutting 4 facets of asmall chip with an angle is a difficult processing, posing a problem incosts.

Furthermore, the light advancing in the vertical direction is alsoassociated with problems in that a standing wave that repeats reflectionbetween an interface of GaN group semiconductor layer/sapphire substrateand an interface of GaN group semiconductor layer/p-type electrode (orsealing material) is formed and the like, which in turn hinderlight-extraction efficiency.

It is a first object of the present invention to provide alight-emitting element having a novel structure capable of solving theabove-mentioned problem, directing the light in the lateral direction,which is produced in the light-emitting layer, to the outside, andfurther, suppressing the occurrence of the above-mentioned standingwave.

In addition to the problem of the light-extraction efficiency asmentioned above, the following problem of lower output is present whenInGaN is used as a material of a light-emitting layer and theultraviolet light is to be emitted.

A light-emitting element comprising InGaN as a light-emitting layergenerally provides highly efficient emission. This is explained to beattributable to a smaller proportion of carriers captured by thenon-radiative center, from among the carriers injected into thelight-emitting layer, due to the localization of the carriers caused byfluctuation of the In composition, which in turn results in a highlyefficient emission.

When a blue purple light—ultraviolet light having a wavelength of notmore than 420 nm is to be emitted by a GaN group light-emitting diode(LED) and a GaN group semiconductor laser (LD), InGaN (In compositionnot more than 0.15) is generally used as a material of a light-emittinglayer, and the structure involved in the emission is a single quantumwell structure (what is called a DH structure is encompassed because ofa thin active layer) or a multiple quantum well structure.

In general terms, the upper limit of the wavelength of the ultravioletlight is shorter than the end (380 nm–400 nm) of the short wavelength ofvisible light, and the lower limit is considered to be about 1 nm (0.2nm–2 nm). In this specification, the blue purple light of not more than420 nm emitted by the above-mentioned InGaN having an In composition ofnot more than 0.15 is also referred to as an ultraviolet light and asemiconductor light-emitting element emitting such ultraviolet light isreferred to as an ultraviolet light-emitting element.

The ultraviolet light GaN can produce has a wavelength of 365 nm.Therefore, in the case of a ternary system wherein InGaN essentiallycontains In composition and free of Al composition, the lower limit ofthe wavelength of the ultraviolet light which can be generated is longerthan the aforementioned 365 nm.

When compared to blue and green light-emitting elements having alight-emitting layer having a high In composition, the ultravioletlight-emitting element has a shorter wavelength. Thus, the Incomposition of the light-emitting layer needs to be reduced. As aconsequence, the effect of the localization of the aforementionedfluctuation of the In composition decreases and the proportion thereofto be captured in the non-radiative recombination center increases,which prevents a high output. Under the circumstances, dislocationdensity, which causes the non-radiative recombination center, has beenactively reduced. As a method for reducing the dislocation density, ELOmethod (lateral growth method) can be mentioned, and high output andlong life have been achieved by reducing the dislocation density (seereference (Jpn. J. Appl. Phys. 39(2000) pp. L647) etc.).

In a GaN group light-emitting element, a light-emitting layer (welllayer) is sandwiched between clad layers (barrier layers) made of amaterial having a greater band gap. According to a reference (HirooYonezu, Hikari Tsushin Soshi Kogaku, Kougakutosho Ltd., p. 72), aguidance of setting the difference in the band gap to generally not lessthan “0.3 eV” has been provided.

From the above-mentioned background, when InGaN having a compositioncapable of emitting ultraviolet light is to be used as a light-emittinglayer (well layer), the clad layer (a single quantum well structurecontains not only a clad layer but also a barrier layer) used tosandwich the light-emitting layer is AlGaN having a greater band gap inview of enclosure of the carrier.

In addition, when a quantum well structure is to be constituted, thebarrier layer needs to have a thickness of a level producing a tunneleffect, which is generally about 3–6 nm.

For example, FIG. 9 shows one embodiment of a conventionallight-emitting diode using In_(0.05)Ga_(0.95)N as a material of alight-emitting layer, which has an element structure wherein an n-typeGaN contact layer 202, an n-type Al_(0.1)Ga_(0.9)N clad layer 203, anIn_(0.05)Ga_(0.95)N well layer (light-emitting layer) 204, a p-typeAl_(0.2)Ga_(0.8)N clad layer 205 and a p-type GaN contact layer 206 aresequentially laminated on a crystal substrate S10 via a buffer layer201, by crystal growth, and a lower electrode (generally an n-typeelectrode) P10 and an upper electrode (generally p-type electrode) P20are formed.

However, the ELO method is problematic in that the methods for growing aGaN layer to be a base, forming a mask layer and re-growing arenecessary, and growth in a number of times is necessary, thus increasingthe number of steps. In addition, because a re-growth interface exists,it has a problem that, although dislocation density reduces, the outputdoes not increase easily.

The present inventors have studied conventional element structures in anattempt to use InGaN as a material of the light-emitting layer andachieve higher output of ultraviolet light, and found that AlGaN layeris behind the distortion relative to the InGaN light-emitting layer,which results from the difference in the lattice constant.

It has been also found that, when a barrier layer is made thin in thequantum well structure, Mg is diffused from the p-type layer formedthereon to a light-emitting layer and forms a non-radiative center, thusproblematically preventing high output of an ultraviolet light-emittingelement.

A second object of the present invention is to achieve high output, andfurther, a long life, by optimizing the structure of the element, whenInGaN is used as a material of a light-emitting layer of thelight-emitting element of the present invention and ultraviolet light isto be emitted.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention is characterized by the following.

-   (1) A semiconductor light-emitting element having an element    structure comprising a first crystal layer processed to have    concaves and convexes on its surface, a second crystal layer    directly formed thereon or formed via a buffer layer, burying the    concaves and convexes, the second crystal layer being made from a    semiconductor material having a different refractive index from the    aforementioned crystal layer, and a semiconductor crystal layer    comprising a light-emitting layer laminated thereon.-   (2) The semiconductor light-emitting element of the above-mentioned    (1), wherein the second crystal layer and the semiconductor crystal    layer thereon are made of a GaN group semiconductor crystal.-   (3) The semiconductor light-emitting element of the above-mentioned    (2), wherein the first crystal layer is a crystal substrate and the    second crystal layer has grown while substantially forming a facet    structure from the concaves and convexes processed on the surface of    the crystal substrate.-   (4) The semiconductor light-emitting element of the above-mentioned    (3), wherein the concaves and convexes processed on the surface of    the crystal substrate have a stripe pattern, and the longitudinal    direction of the stripe is a <11-20> direction or a <1-100>    direction of a GaN group semiconductor grown while burying them.-   (5) The semiconductor light-emitting element of the    above-mentioned (1) or (4), wherein the concaves and convexes have a    cross sectional shape of a rectangular wave, a triangular wave or a    sine curve.-   (6) The semiconductor light-emitting element of the above-mentioned    (1), wherein the difference between the refractive index of the    first crystal layer and that of the second crystal layer at the    wavelength of a light emitted from the light-emitting layer is not    less than 0.05.-   (7) The semiconductor light-emitting element of the above-mentioned    (1), wherein the light-emitting layer is made of an InGaN crystal    having a composition capable of generating an ultraviolet light.-   (8) The semiconductor light-emitting element of the above-mentioned    (1), wherein the light-emitting layer is a quantum well structure    comprising a well layer made of InGaN and a barrier layer made of    GaN.-   (9) The semiconductor light-emitting element of the above-mentioned    (1), wherein the first crystal layer is a crystal substrate, the    second crystal layer is grown, via a low temperature buffer layer,    on the concaves and convexes processed on the surface of the crystal    substrate while burying the concaves and convexes, the    light-emitting layer is a quantum well structure comprising a well    layer made of InGaN and a barrier layer made of GaN, and all the    layers between the quantum well structure and the low temperature    buffer layer are made of a GaN crystal.-   (10) The semiconductor light-emitting element of the    above-mentioned (8) or (9), wherein the barrier layer has a    thickness of 6 nm–30 nm.-   (11) A semiconductor light-emitting element having an element    structure comprising a crystal layer surface to be a base for    crystal growth, a first GaN group semiconductor crystal grown on    said surface to form concaves and convexes, a second GaN group    semiconductor crystal having a different refractive index from the    first GaN group semiconductor crystal, which is grown while covering    at least a part of said concaves and convexes, a third GaN group    semiconductor crystal grown until it flattens the above-mentioned    concaves and convexes, and a semiconductor crystal layer having a    light-emitting layer laminated thereon.-   (12) The semiconductor light-emitting element of the above-mentioned    (11), wherein the crystal layer surface to be a base for crystal    growth has a structure or has been subjected to a surface treatment,    which dimensionally limits a crystal growth area, and said    limitation causes growth of the first GaN group semiconductor    crystal into concaves and convexes, while forming substantial facet    structure or a pseudo-facet structure.-   (13) The semiconductor light-emitting element of the above-mentioned    (12), wherein the structure or surface treatment limiting the    crystal growth area is concaves and convexes processed on the    crystal layer surface to be a base for crystal growth, or a mask    pattern capable of causing a lateral growth, which is formed on the    surface of the crystal layer to be a base for crystal growth, or a    surface treatment capable of suppressing GaN group crystal growth,    which is applied to a specific area of the surface of the crystal    layer to be a base for crystal growth.-   (14) The semiconductor light-emitting element of the above-mentioned    (11), having an element structure wherein the second GaN group    semiconductor crystal is grown covering, in a membrane, at least a    convex part of the concaves and convexes made of the first GaN group    semiconductor crystal, the third GaN group semiconductor crystal    covering same is grown until it flattens the above-mentioned    concaves and convexes, and the semiconductor crystal layer having    the light-emitting layer is laminated thereon, and wherein the    second GaN group semiconductor crystal has a multilayer membrane    structure.-   (15) The semiconductor light-emitting element of the above-mentioned    (11), wherein the light-emitting layer is made of an InGaN crystal    having a composition capable of generating an ultraviolet light.-   (16) The semiconductor light-emitting element of the above-mentioned    (11), wherein the light-emitting layer is a quantum well structure    comprising a well layer made of InGaN and a barrier layer made of    GaN.-   (17) The semiconductor light-emitting element of the above-mentioned    (16), wherein the barrier layer has a thickness of 6 nm–30 nm.-   (18) The semiconductor light-emitting element of the above-mentioned    (11), wherein the above-mentioned concaves and convexes have a    stripe pattern, and the longitudinal direction of the stripe is a    <11-20> direction or a <1-100> direction of the first GaN group    semiconductor crystal.

In the following, the embodiment of the above-mentioned (1) is referredto as “embodiment (I)” and the embodiment of the above-mentioned (11) isreferred to as “embodiment (II)” and explained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing an example of the structure of thelight-emitting element of the present invention, wherein hatching ispartially applied to show the boundary of areas (the same in thefollowing Figures).

FIG. 2 is a schematic drawing showing one example of the crystal growthmethod for forming a concavo-convex refractive index interface in theembodiment (I) of the present invention.

FIG. 3 is a schematic drawing showing a method of processing the surfaceof a crystal substrate to form concaves and convexes having a slant inthe embodiment (I) of the present invention.

FIG. 4 is a schematic drawing showing one example of the crystal growthmethod for forming a concavo-convex refractive index interface in theembodiment (II) of the present invention.

FIG. 5 is a schematic drawing showing another example of the crystalgrowth method for forming a concavo-convex refractive index interface inthe embodiment (II) of the present invention.

FIG. 6 is a schematic drawing showing a variation of the crystal growthmethods shown in FIGS. 4 and 5.

FIG. 7 is a schematic drawing showing another example of the crystalgrowth method for forming a concavo-convex refractive index interface inthe embodiment (II) of the resent invention.

FIG. 8 is a schematic drawing showing the structure of a conventionalGaN group light-emitting element.

FIG. 9 is a schematic drawing showing one example of a conventionallight-emitting diode using In_(0.05)Ga_(0.95)N as a material of alight-emitting layer.

DETAILED DESCRIPTION OF THE INVENTION

Since the problem of the present invention has the most importantsignificance for a light-emitting element, the light-emitting element ofthe present invention is most preferably in the form of an LED. Whilethe materials are not limited, an LED using a GaN group material. (GaNgroup LED), wherein the usefulness of the present invention becomesparticularly remarkable as shown below, is taken as an example, and suchlight-emitting element is explained.

The light-emitting element enhances, in any embodiment, thelight-extraction efficiency by the action and effect of a concavo-convexrefractive index interface formed downward of the light-emitting layer.The light-emitting element can be further grouped into theabove-mentioned embodiment (I) and embodiment (II) based on how thisconcavo-convex refractive index interface is formed.

In the above-mentioned embodiment (I), concaves and convexes areprocessed on a crystal substrate and buried with a semiconductor crystal(particularly GaN group crystal) to constitute a concavo-convexrefractive index interface.

In the above-mentioned embodiment (II), a GaN group crystal is grown inthe concaves and convexes, which are buried with a different GaN groupcrystal to constitute a concavo-convex refractive index interface.

First, the above-mentioned embodiment (I) is explained. FIG. 1( a) showsa GaN group LED as an example of the structure of the light-emittingelement of embodiment (I), wherein concaves and convexes 1 a areprocessed on the surface of the first crystal layer (hereinafter to bealso referred to as the “first layer”) 1, and a second crystal layer(hereinafter to be also referred to as the “second layer”) 2 made of amaterial having a refractive index different from that of theaforementioned crystal layer is grown directly or via a buffer layer,while burying the concaves and convexes. As a result, an interfacehaving a different refractive index is formed in a concavo-convex form.Still thereon is laminated a semiconductor crystal layer (n-type contactlayer 3, light-emitting layer A, p-type contact layer 4) by crystalgrowth, and electrodes P1 and P2 are formed to give an elementstructure. While the element structure in this Figure is a simple DHstructure, an exclusive contact layer, an exclusive clad layer and thelike may be formed, and the light-emitting layer may be an SQW structureor MQW structure, possibly having any structure as a light-emittingelement.

Due to the above-mentioned constitution, the light produced in thelight-emitting layer A, which propagates in the lateral direction, isinfluenced by a concavo-convex refractive index interface 1 a, whichcauses a kind of mode conversion (change in the light direction to thedirection of surface emission), and advances in a direction other thanthe lateral direction. As a result, the amount of the light headingtoward the extraction surface increases, and the light absorption layerin the element decreases. As a result, the light-extraction efficiencyis improved.

As stated in the description of the prior art, the light that advancesin the direction other than toward the extraction outlet of the light(e.g., downward direction and lateral direction) has been conventionallymade to head toward the outlet solely by simple reflection of the lightby an end surface.

In contrast, in the present invention, a GaN group semiconductor layerregion formed on a substrate by epitaxial growth is regarded [awaveguide that propagates light in the lateral direction], and along thewaveguide, a concavo-convex refractive index interface is formed at aposition capable of affecting the light directed to the lateraldirection, thereby causing a kind of mode conversion (or diffusedreflection) to direct the light toward a different direction.

The present invention takes note of the fact that the light propagatingin the lateral direction propagates in the lateral direction as anelectromagnetic wave having an electric field widely expanded to theupper and lower layers thereof, with the light-emitting layer as acenter. The thickness of the light-emitting layer is approximately 10nm–100 nm for the active layer of a typical DH structure. The light inthe lateral direction propagates not only within such a thin activelayer but also in the lateral direction as a wave having a widedistribution width reaching the crystal substrate. As shown in FIG. 1(a), therefore, when a concavo-convex refractive index interface 1 a isformed within the light distribution in the lateral direction, the waveof the light in the lateral direction is influenced and a certain amountthereof can be directed to a different direction by a sort of modeconversion (or by producing a diffused reflection), which in turnincreases the amount of light that goes outside. These concaves andconvexes also function as a reflection surface that causes upwardlydiffused reflection for the light emitted from the light-emitting layertoward the concaves and convexes per se.

Moreover, these concaves and convexes also function to lower thereflectance, in the perpendicular direction, of the interface of GaNgroup semiconductor layer/sapphire substrate. Thus, it is possible tosuppress occurrence of a standing wave in the vertical direction,thereby to allow a large amount of light to enter the sapphiresubstrate, increase the amount of light to be extracted from thesapphire substrate, and enhance the light-extraction efficiency,particularly when extracting the light from the substrate side.

In embodiment (I), the concaves and convexes to be processed on thesurface of a first layer are those formed by the surface itself of thefirst layer. They are different from the concaves and convexes formedwhen a mask layer made from SiO₂ and the like, used for theconventionally known lateral growth method, is applied to a flatsurface.

In addition, the above-mentioned constitution makes it possible topreferably reduce the dislocation density of the GaN group crystal grownon a crystal substrate. By this constitution, dislocation density can bereduced by a one time growth without using a mask layer for ELO.

Namely, by the ELO method using a mask, a GaN film is grown on a base,once taken out from the growth apparatus to form a mask, then returnedto a growth apparatus for re-growth. In contrast, by a growth methodcomprising formation of concaves and convexes on a crystal substrate,the growth does not need to be stopped once a crystal substrate afterconcave-convex processing is set in a growth apparatus. As a result, are-growth interface is void and a crystal having fine crystallinity canbe produced.

According to the above-mentioned constitution of the present invention,moreover, since a GaN group crystal layer is grown without using a mask,the problems of staining with impurities due to decomposition of themask and degradation of crystal quality are obliterated.

These actions and effects afford a fine crystal having less dislocation,thus strikingly increasing the emission output. In addition, since thedislocation density that causes degradation reduces, a longer life canbe obtained.

The whole location pattern of concaves and convexes may be any as longas it can exert an influence on the wave of light in the lateraldirection, and may be a pattern wherein dot-like concave parts (orconvex parts) are sequenced on the surface (standard plane) of the firstlayer, or a stripe like concavo-convex pattern wherein linear or curvedconcave grooves (or convex ridges) are sequenced at certain intervals. Apattern of convex ridges arranged in a lattice shape can be said to bean arrangement of angular concave parts. Of these, a stripe likeconcavo-convex pattern can exert a strong influence on the light in thelateral direction.

The cross sectional shape of the concaves and convexes may be arectangular (including trapezoid) wave as shown in FIG. 2( a), atriangular wave and a sine curve as shown in FIG. 3( c), a combined waveof these and the like.

For the specification of the details of the concaves and convexes, theconcavo-convex structure for crystal growth, which is formed for thereduction of dislocation density of the GaN group crystal to bementioned later, may be referred to.

Furthermore, for the concaves and convexes to exert an influence on thelight in the lateral direction, the concaves and convexes are preferablywithin a specific distance from the light-emitting layer. This distanceis about 0.5 μm–20 μm, particularly preferably 1 μm–10 μm, shown by k inFIG. 1( a), and includes the distance between the upper surface of thesubstrate and the undersurface of the light-emitting layer in generalLEDs. Thus, when an element structure is constructed by using a crystalsubstrate of the element as a first layer, forming concaves and convexesthereon, and growing a second layer to bury them, the concaves andconvexes sufficiently exert an influence on the light in the lateraldirection.

The materials of the light-emitting element may be conventionally knownmaterials such as GaAs group, InP group, GaN group and the like. Theusefulness of the present invention becomes highly remarkable in a GaNgroup light-emitting element (at least a material of a light-emittinglayer being a GaN group semiconductor) having a major problem ofreduction of dislocation density of crystal. In a GaN grouplight-emitting element, reduction of dislocation density of GaN groupcrystal is an essential assumption for forming an element. In thepresent invention, reduction of dislocation density of GaN group crystalis achieved by providing a growth method using a useful concavo-convexstructure as in the following. Because the concavo-convex structure canserve both as the concaves and convexes of the above-mentionedrefractive index interface, the usefulness of the concaves and convexescan be enhanced as compared to concaves and convexes formed only for thepurpose of refractive index interface. In the following, a GaN groupcrystal growth method using such concavo-convex structure is explained.

The GaN group crystal growth method using a concavo-convex structurecomprises, as shown in FIG. 2( a), processing concaves and convexes 1 aon the surface of a crystal substrate (first layer) 1, and growing, asshown in FIG. 2( b), GaN group crystals 21, 22 from the concave partsand convex parts thereof while substantially forming a facet structure,whereby, as shown in FIG. 2( c), said concaves and convexes aresubstantially buried with the GaN group crystal, thus burying themwithout leaving concave parts as cavities. By the growth whilesubstantially forming a facet structure is meant the growth of the typeof facet structure growth to be mentioned later (e.g., growth whileforming concaves and convexes in the thickness direction and the like).In the following, this growth method using concaves and convexes andburying the concave parts is referred to as “said facet growth method”.

The said facet growth method to be used in the present invention ischaracterized in that a base surface where a facet plane can be formedis provided in advance from the early stage of the crystal growth, byprocessing concaves and convexes on the surface of a crystal substratewhen a buffer layer etc. have not been formed yet.

By forming concaves and convexes on a crystal substrate, concavesurfaces and convex surfaces divided by the difference in the levels ofeach other are used as a unit standard plane where facet structuregrowth is performed for vapor phase growth of a GaN group crystal onthis plane. By using both concave surfaces and convex surfaces as planescapable of facet structure growth, as shown in FIG. 2( b), a crystalgrowth forming convexes from both the concave surfaces and convexsurfaces occur in the early stage of the growth.

As a result, the dislocation line extending from the crystal substratein the C-axis direction is bent by a facet plane (shown in FIG. 2( b), aslant plane of crystals 21, 22) into the lateral direction and does notpropagate upward. Then as shown in FIG. 2( c), the growth is continued.When the growth plane is flattened, the vicinity of the surface becomesa low dislocation density region where propagation of dislocation fromthe substrate is reduced.

According to a general method for growing a GaN group crystal, a GaNmembrane is grown on a sapphire C plane substrate via a low temperaturebuffer layer such as AlN and the like at a high temperature by MOVPEmethod and the like. When a high temperature GaN is grown on a lowtemperature buffer layer, a high temperature GaN crystal startsisland-like growth with a part of the crystallized buffer layer as agrowth nucleus, and a crystal having a high growth speed covers andjoins a crystal having a slow growth speed, thereby promoting the growthin the lateral direction to form a flat GaN crystal in time. Whenconcaves and convexes are not processed on a sapphire substrate, sincethe growth proceeds in such a manner that a C plane is produced, whichis stable and has a lower growth speed, the plane is flattened. This isbecause the growth speed in the lateral direction is fast as compared tothe growth speed of the stable C plane.

In contrast, by processing concaves and convexes on a substrate plane, adimensional limitation on the crystal growth region works on the growthin the lateral direction. Therefore, for example, when the longitudinaldirection of the concaves and convexes constitutes a stripe shapeparallel to the <11-20> direction, a limitation is applied to the growthin the <1-100> direction. As a result, the growth rate in the C-axisdirection increases and a slant facet having a slow crystal growth speedsuch as stable {1-101} and the like can be formed. In the presentinvention, by processing concaves and convexes on the growth-plane of asubstrate, a dimensional limitation is applied on the above-mentionedgrowth region in the lateral direction.

In this specification, the crystal plane and crystal orientation to beindicated are all crystal plane and orientation of the GaN crystal grownon a crystal substrate.

By the concave part being substantially buried with the second layermeans not only a completely buried state but any mode of burying as longas it affords an effective concavo-convex refractive index interfacecapable of achieving the object of the present invention. For example, aclearance may be formed at a part where a crystal grown from the concavepart and a crystal grown from the convex part are joined, but it isconvenient because changes in the refractive index can be achieved. Evenwhen a clearance is formed on the concave part, too, it is acceptable aslong as the undersurface of the second layer grown on the concave partenters the concave part to the extent that the object of the presentinvention can be achieved, and an effective concavo-convex refractiveindex interface is constituted.

Besides said facet growth method, for example, JP-A-2000-106455discloses a method for growing a gallium nitride semiconductor, whichcomprises forming concaves and convexes on a crystal substrate andleaving the concave part as a cavity. However, because such growthmethod retains concave parts as cavity without filling, the refractiveindex interface when seen from the second layer (i.e., undersurface ofthe second layer) does not form sufficient concaves and convexes and theaction and effect of mode modulation on the light in the lateraldirection is small. Moreover, the presence of cavity is disadvantageousin releasing the heat generated in the light-emitting layer toward thesubstrate side. In addition, since propagation of the dislocation is notpositively controlled, the dislocation is propagated in the upper partof the convex part, and a dislocation density reduction effect isinsufficient.

The crystal substrate to be used in said facet growth method is a basesubstrate for the growth of various semiconductor crystal layers, and inthe state prior to the formation of buffer layer etc. for latticematching. As preferable crystal substrate, sapphire (C plane, A plane, Rplane), SiC (6H, 4H, 3C), GaN, AlN, Si, spinel, ZnO, GaAs, NGO and thelike can be used, but other materials may be used as long as the objectof the invention can be achieved. The plane orientation of the substrateis not particularly limited and may be a just substrate or a substratehaving an off-angle.

The GaN group semiconductor is a compound semiconductor represented byIn_(x)Ga_(y)Al_(z)N (0≦X≦1, 0≦Y≦1, 0≦Z≦1, X+Y+Z=1). The crystal mixingratio is optionally determined, and, for example, AlN, GaN, AlGaN, InGaNand the like can be mentioned as important compounds.

The concaves and convexes to be used for said facet growth methodpreferably have a concavo-convex shape that permits facet structuregrowth from both of the concave surface and convex surface, as mentionedabove, and capable of acting on the light in the lateral direction,produced in the light-emitting layer. The preferable pattern formed bythe concaves and convexes and the preferable mode of the concaves andconvexes are explained in the following.

For the location pattern of concaves and convexes to be used in thefacet growth method, a general reference may be made to theabove-mentioned concaves and convexes capable of exerting an influenceon the wave of the light in the lateral direction, and may be a patternwherein dot-like concave parts (or convex parts) are sequenced, or astripe like concavo-convex pattern wherein linear or curved concavegrooves (or convex ridges) are sequenced at certain intervals. The crosssectional shape of the concaves and convexes may be a rectangular(inclusive of trapezoid) wave, triangular wave, sine curve and the like,and the pitch may not necessarily be the same, as mentioned above.

Of such various embodiments of concaves and convexes, a stripe likeconcavo-convex pattern wherein linear or curved concave grooves (orconvex ridges) are sequenced at certain intervals is preferable becauseit can simplify the production step thereof, the pattern can be madeeasily, and as mentioned above, an influence on the light in the lateraldirection is significant.

When the pattern of the concaves and convexes is a stripe, thelongitudinal direction of the stripe may be optional. When it is set tobe the <11-20> direction for the GaN group crystal grown while buryingthe concaves and convexes, a slant facet of the {1-110} plane and thelike are tend to be easily formed when a dimensional limitation isapplied on the growth in the lateral direction. As a result, thedislocation propagated in the C-axis direction from the substrate sideis bent by this facet plane into the lateral direction, propagation tothe upward becomes difficult and a low dislocation density region can beformed, which is particularly preferable.

In contrast, when the longitudinal direction of the stripe is set to the<1-100> direction, the effect as mentioned above can be achieved bydetermining the growth conditions that permit easy formation of apseudo-facet plane.

Said facet growth method and preferable size of the concaves andconvexes capable of effectively influencing the direction of the lightin the lateral direction are shown in the following taking concaves andconvexes having a cross section of rectangular wave as shown in FIG. 2(a).

The width W1 of the concave groove is preferably 0.5 μm–20 μm,particularly 1 μm–10 μm.

The width W2 of the convex part is preferably 0.5 μm–20 μm, particularly1 μm–10 μm.

The amplitude of concaves and convexes (depth of concave groove) d is0.05 μm–5 μm, particularly preferably 0.2 μm–3 μm.

These sizes and the pitch etc. calculated therefrom are the same asthose of the concaves and convexes having different cross sectionalshape.

While how a facet plane is grown on a GaN group crystal varies dependingon the combination of the width of a concave part and the width of aconvex part, this facet plane suffices for use as long as it can bentthe propagation of dislocation preferable embodiment is as shown in FIG.2( b), wherein each crystal unit 21, 22 grown from the unit standardplane form a mountain shape with both facet planes crossing at the topwithout forming a flat part on each top (long ridges in the shape oftriangle or a mountain range). With such facet plane, almost all thedislocation lines continued from the aforementioned base plane can bebent and the dislocation density immediately above can be furtherreduced.

Note that not only by the combination of the widths of concaves andconvexes but also by changing the depth of the concave part (height ofconvex part) d, the facet plane forming area can be controlled.

As the processing method of concaves and convexes, for example, a methodcomprising patterning according to the embodiment of the objectiveconcaves and convexes by general photolithographic techniques, andetching processing by RIE technique and the like to give the objectiveconcaves and convexes, and the like can be mentioned.

The method for growing a semiconductor crystal layer on a substrate ispreferably HVPE, MOVPE, MBE method and the like. When a thick film is tobe prepared, HVPE method is preferable and a thin film is to beprepared, MOVPE method and MBE method are preferable.

The formation of a facet plane can be controlled by the growthconditions (kind of gas, growth pressure, growth temperature and thelike) for crystal growth. By growth under reduced pressure with lowerNH₃ partial pressure, a {1-101} plane facet is easily formed and facetplanes can be easily formed by normal pressure growth as compared togrowth under reduced pressure.

When the growth temperature is raised, the growth in the lateraldirection is promoted. By low temperature growth, the growth in theC-axis direction becomes faster than that in the lateral direction and afacet plane is easily formed.

The above indicates that control of a facet shape is available dependingon the growth conditions. As long as the effect of the present inventionis achieved, the conditions can be determined according to the object.

In said facet growth method, when a GaN group crystal is to be grownfrom the concaves and convexes formed on a crystal substrate, it may bedirectly grown on the crystal substrate, or may be grown via a known lowtemperature buffer layer such as GaN, AlN and the like, or other knownbuffer layer.

While the above shows a method of burying concaves and convexes by saidfacet growth method, concaves and convexes may be buried by generalgrowth (e.g., growth for greater growth in the lateral direction)without focusing on a facet structure growth, by selecting the size andcrystal growth conditions of the concaves and convexes.

The next exemplifies an embodiment wherein the cross section of concavesand convexes is a triangular wave. This embodiment is particularlyuseful when a GaN crystal substrate is used as a first layer.

As a method for processing the surface of a crystal substrate intoconcaves and convexes having such inclination, for example, a methodcomprising forming a resist R having a cross sectional shape of a convexarch with thinner both edges in an object pattern of a stripe, a latticeand the like, on the surface of a GaN substrate 1, and applying the gasetching, as shown in FIG. 3( a), can be mentioned. As a material of theresist, one capable of the gas etching is preferably used. By applyingthe gas etching to a GaN substrate with a resist R, the exposed area ofthe GaN substrate can be eroded from the first, and the thin shoulder ofthe resist is ablated as the etching proceeds, and the etching of theGaN crystal begins belatedly. Because the starting time of the etchingis postponed in this way, concaves and convexes ultimately have a neartriangular wave cross section as a whole, as shown in FIG. 3( b). Thethickest part of the resist may be removed by said gas etching but maybe retained. In this case, a remover exclusive for the resist, which isfree of damage to the GaN crystal, may be used to remove this part. Inaddition, an etching treatment of the convex part is more effectivelyconducted finally.

Preferable sizes of the concaves and convexes having inclination asshown in FIG. 3( b) are as follows.

The pitch of the concaves and convexes is 2 μm–40 μm, particularlypreferably 2 μm–20 μm.

The amplitude of the concaves and convexes is 0.05 μm–5 μm, particularlypreferably 0.2 μm–3 μm.

The arrangement pattern of the concaves and convexes having aninclination is, as in said facet growth method explained in the above, apattern wherein dot-like concave parts (or convex parts) are sequenced,or a stripe like concavo-convex pattern wherein linear or curved concavegrooves (or convex ridges) are sequenced at certain intervals. A stripeconcavo-convex pattern is particularly preferable.

As shown in FIG. 3( c), the growth of the second layer 2 is started fromthe entire surface of the concaves and convexes, and the growth iscontinued until the concaves and convexes are completely buried. Sincethe side wall of the concave groove is a pseudo-facet plane at thispoint, the action and effect is achieved that a dislocation line is bentwith said facet plane as an interface when a GaN group crystal is grown,and a part with a low dislocation density can be formed in the upperlayer. In addition, such concaves and convexes not only act on the lightin the lateral direction but also act as a reflection surface, which isa preferable embodiment.

While the etching method is not limited, gas etching by RIE (ReactiveIon Etching) and the like using an etching gas including chlorine ispreferable because it does not damage a crystal surface when the firstlayer is a GaN crystal substrate.

In the above description, the concavo-convex structure of said facetgrowth method is also used as concaves and convexes for the light in thelateral direction in a GaN group light-emitting element. However, suchconcurrent use is not essential and concaves and convexes only for thelight in the lateral direction may be separately formed.

Now the above-mentioned embodiment (II) is explained. FIG. 1( b) shows aGaN group LED as an example of the structure of the light-emittingelement by the above-mentioned embodiment (II), wherein a first GaNgroup crystal (hereinafter to be also referred to as “first crystal”) 10is grown on a surface of the crystal layer (crystal substrate in thisFigure) S to be the base of the crystal growth, while forming concavesand convexes with a facet structure, a second GaN group crystal(hereinafter to be also referred to as a “second crystal”) 20, having adifferent refractive index from the first GaN group crystal, is growncovering, from among said concaves and convexes, at least convex parts(first crystal 10 itself in the embodiment of FIG. 4), therebyconstituting a concavo-convex refractive index interface 10 a and thesame action and effect as in the above-mentioned embodiment (I) can beachieved.

In this embodiment (II), it is important that the composition is changedto a different GaN group crystal when the first crystal is grown to formconcaves and convexes, thereby changing the refractive index, namely,not to allow growth of the first crystal to the point it flattens. Thechanges in the refractive index (changes in composition) may bestep-like changes or continuous changes as shown in the distributionrefractive index waveguide.

The method of growing the first crystal into concaves and convexes isnot limited. By the growth while substantially forming a facetstructure, or while forming a pseudo-facet structure, the concaves andconvexes can be grown, which preferably achieves the object of thepresent invention.

The concaves and convexes as used herein are not only those whereinconvex parts are successively adjoined to form a wave, but also thosewherein convex-like first crystals 10 are disposed dispersingly and adifferent substance is positioned as a concave part between them, asshown in FIG. 5( a)–(c).

The shape of the concaves and convexes formed by the facet growth of thefirst crystal is not limited, and, for example, may be a trapezoidhaving a flat part on the top of the convex part. To achieve sufficientaction and effect of the concavo-convex refractive index interface, asexplained in the above-mentioned embodiment (I), an embodiment wherein acrystal unit grown from each unit standard plane forms a mountain shapewith both facet planes crossing at the top without forming a flat parton each top (long ridges in the shape of triangle or a mountain range)is preferable.

In embodiment (II), any method can be used as long as it can formconcaves and convexes with the first crystal, and when the first crystalforms concaves and convexes, a second crystal is grown to cover them,thereby constituting a concavo-convex refractive index interface.

A method of growing a GaN group crystal in concaves and convexes isparticularly preferably a method for achieving facet growth (or growthanalogous thereto). For this end, a method dimensionally limiting thecrystal growth region to the crystal layer surface to be a base forcrystal growth can be mentioned.

For example, (1) a method of processing concaves and convexes on thecrystal layer surface to be a base for crystal growth as the said facetgrowth method explained in detail above (FIG. 1( b), FIG. 4, FIG. 5( a),FIG. 6, FIG. 7), (2) a method wherein a mask pattern that preventsgrowth of a GaN group crystal is formed in a specific region of thecrystal layer surface to be a base for crystal growth (FIG. 5( b)), (3)a method wherein a surface treatment for suppressing GaN group crystalgrowth is applied to a specific region of the crystal layer surface tobe a base for crystal growth (FIG. 5( c)) and the like can be mentioned.

By these methods, the first crystal is grown to form concaves andconvexes.

As a method of the above-mentioned (1), not only an embodiment whereinthe concave part of the concaves and convexes is substantially buriedwith GaN group crystals 10, 20 based on said facet growth method, asshown in FIG. 4, but also an embodiment as shown in FIG. 5( a), whereina first crystal 10 is facet grown exclusively from the upper part of theconvex part, then a second crystal 20 is employed to be laterally grownon the concave part, leaving the concave part as a cavity may beemployed. It is possible to utilize concaves and convexes having aninclination explained as an embodiment of FIG. 3, in the above-mentionedembodiment (I). This is an embodiment wherein, as shown in FIG. 7, afirst crystal 10 is grown on concaves and convexes having an inclinationon a crystal substrate S to perform a pseudo-facet growth, and thenchanged to a second crystal 20.

As a method of the above-mentioned (2), various lateral growth methodsusing a conventionally known mask are all applicable, as shown in FIG.5( b).

As a material of the mask m, a known mask material may be used, such asnitrides and oxides of Si, Ti, Ta, Zr and the like, namely, SiO₂,SiN_(x), TiO₂, ZrO₂ etc. As a mask pattern, a known pattern may bereferred to but mainly, a stripe pattern, a lattice pattern and the likeare important, and the direction of the boundary between the mask regionand the non-mask region is particularly important. When the boundarybetween the mask region and the non-mask region is to be the straightline extending in the <1-100> direction of the GaN group crystal to begrown, the growth in the lateral direction becomes faster. Conversely,when the boundary between the mask region and the non-mask region is tobe the <11-20> direction, a slant facet such as {1-101} plane and thelike is easily formed, which is preferable facet growth for the presentinvention.

For a specific size of a mask, an atmospheric gas (H₂, N₂, Ar, He etc.),crystal growth method (HVPE, MOVPE) and the like for performing alateral growth method using a mask, known techniques may be referred to,and, for example, the detail is described in references (A. Sakai etal., Appl. Phys. Lett. 71(1997) 2259.).

As the method of the above-mentioned (3), for example, a methoddescribed in JP-A-2000-277435, which uses an SiO₂ residue as a mask canbe mentioned. By this method, the action and effect similar to those inthe above-mentioned mask can be achieved and a GaN group crystal 10 canbe facet grown in a convex shape from an area free of the treatment.

As the combination (first crystal/second crystal) of a first crystal tobe grown in a convex shape and a second crystal covering same in theabove-mentioned embodiment (II), (AlGaN/GaN), (AlInGaN/GaN) and the likecan be mentioned. By the presence as a first crystal of AlGaN downsideof GaN, a second crystal GaN corresponds to a core having a highrefractive index as referred to in the optical waveguide, the firstcrystal AlGaN corresponds to a clad having a lower refractive index thanthe above, enhancing the action and effect of the present invention, andeffectively acts as a reflective layer. The GaN group crystal (e.g.,GaN) that buries concaves and convexes may be undoped or of an n-type.

While (1) to (3) above are various methods to facet grow a GaN groupcrystal, a third GaN group crystal to flatten concaves and convexes inany method may be a second crystal (embodiment wherein second crystalsis successively grown until it flattens), or a crystal (including firstcrystal) different from a second crystal. In addition, the third GaNgroup crystal may be varied to comprise multi layers.

There exists a common variation to change the composition of the GaNgroup crystal to a multi-layered one during or after the growth of thefacet structure by the selection of the mode of the third GaN groupcrystal. Such variation is explained in the following taking theformation of concaves and convexes by said facet growth method of theabove-mentioned (1) as an example.

In the embodiment of FIG. 4( a), the second crystal 20 that covers thefirst crystal 10 continues to grow until it flattens the concaves andconvexes. In this variation, the second crystal (e.g., AlGaN) 20covering the first crystal (e.g., GaN) 10 is a membrane and grows untila GaN group crystal (e.g., GaN) 20 a having a different refractive indexis flattened, as shown in FIG. 4( b). Further, in the embodiment of FIG.4( c), a second crystal 20 grows as a membrane covering the firstcrystal 10 and a first crystal 20 a and a second crystal 20 bsequentially cover the second crystal 20, thus forming a multilayerstructure of GaN group crystal membranes having different refractiveindices, and grows until a GaN group crystal 20 c is flattened, as shownin FIG. 4( c).

By an embodiment of a multilayer structure consisting of GaN groupcrystal membranes having different refractive indices, reflectingproperty can be further improved. For example, a Bragg reflective layermay be formed by optimally designing the membrane thickness relative tothe emission wavelength and affording an ultra lattice structure by apair of AlGaN/GaN and the like.

When a multilayer structure is to be formed, the number of layers of themembrane is not limited and, as shown in FIG. 4( b), it may be astructure sandwiching one layer of membrane, a structure comprisingmulti-layers (5 pairs–100 pairs) as shown in FIG. 4( c), and the like.

When to change the first crystal grown (particularly preferably facetgrown) on the concaves and convexes to a second crystal is not limited,and the composition may be changed from the initial growth stage ofgrowth on the concaves and convexes formed on the substrate S, as, forexample, schematically shown in FIG. 6 which shows the growth state ofthe multi-layer concaves and convexes made of a GaN group crystal. Inthis Figure, hatching is applied to distinctively show that a GaN groupcrystal having a different refractive index grows in a multi-layer stateto form concaves and convexes.

In embodiment (II), a concavo-convex refractive index interfacepreferably have a height of the convex part of 0.05 μm–10 μm,particularly 0.1 μm–5 μm, to preferably achieve the object of thepresent invention. In addition, the pitch of the concavo-convexrefractive index interface is generally 1 μm–10 μm, particularlypreferably 1 μm–5 μm, by a conventionally known lateral growth method.The pitch of the concaves and convexes obtained by said facet growthmethod is the same as that in the above-mentioned embodiment (I).

In either the above-mentioned embodiment (I) or (II), the differencebetween the refractive index of the first layer (first crystal) and thatof the second layer (second crystal) is preferably not less than 0.01,particularly not less than 0.05, at the wavelength of the light emittedfrom the light-emitting layer.

The relationship between various refractive indices of the both ispreferably first layer (first crystal)<second layer (second crystal),whereby the second layer (second crystal) corresponds to the core of thehigh refractive index as referred to in the optical waveguide, the firstlayer (first crystal) corresponds to the clad having a lower refractiveindex, and the action and effect of the present invention can beenhanced further.

The next shown is a preferable embodiment using InGaN as a material of alight-emitting layer and the ultraviolet light (wavelength 420 nm orbelow) is output. The InGaN in this case has an In composition of notmore than 0.15.

In both the above-mentioned embodiments (I) and (II), a fine crystalhaving less dislocation due to concaves and convexes can be formed,which in turn strikingly increases the emission output. In addition, thedislocation density that causes degradation is reduced and a long lifecan be achieved.

A preferable embodiment when the ultraviolet light is output is theabove-mentioned embodiment (I) wherein the material of the GaN groupcrystal layer to be formed on the concaves and convexes of the substrateis limited to a GaN crystal. An MQW structure comprising an InGaNcrystal layer having a composition permitting generation of ultravioletlight as a well layer is constituted on this GaN crystal layer and usedas a light-emitting layer. In other words, this is a constitutionwherein the n-type clad layer consists of GaN and an AlGaN layer doesnot exist between the light-emitting layer and the low temperaturebuffer layer.

In this embodiment, an InGaN having a composition permitting generationof ultraviolet light is used as a light-emitting layer, whereas as amaterial of an n-type clad layer, AlGaN, which is conventionallyconsidered to be essential, is not used but GaN is used. In the presentinvention, it has been found with regard to the ultravioletlight-emitting layer, that even if an n-type clad layer is GaN, positiveholes can be sufficiently confined. This is considered to beattributable to the fact that, since the effective mass of the positivehole injected from the p-type layer is large, it has a short diffusionlength and cannot sufficiently reach the n-type clad layer. In theconstitution of the present invention, therefore, the n-type GaN layerthat is present as a lower layer of the InGaN light-emitting layer doesnot correspond to the conventional clad layer in a strict sense. Byremoving the AlGaN present as a clad layer between the crystal substrateand the light-emitting layer and employing a GaN layer, the distortionof the InGaN light-emitting layer can be reduced.

When distortion is applied to the light-emitting layer (well layer), apiezo-electric field is generated due to the distortion, which causestilting of the well structure, thereby reducing the overlap in wavefunction of electron and hole. As a result, recombination probability ofelectron and hole decreases and the emission output is weakened. Toavoid this, an attempt has been made to dope Si in the MQW structurethereby to cancel the piezo-electric field. However, since crystallinityis degraded due to the doping, this is not a preferable method. Byremoving an n-type AlGaN layer, as mentioned above, such possibility isobliterated and high output can be achieved.

Combined with the reduction of the dislocation density using concavesand convexes of a substrate explained above and the above-mentionedaction and effect by removing AlGaN, dislocation density of the InGaNlight-emitting layer is reduced, as well as the distortion is reduced,which in turn sufficiently improves emission output and the life of anelement.

In another preferable embodiment for output of ultraviolet light, thematerial of the barrier layer of a quantum well structure of thelight-emitting layer is limited to GaN. As a result, an AlGaN layerbetween a well layer and a low temperature buffer layer is removed,distortion of a well layer is suppressed and high output and long lifecan be achieved. In a conventional quantum well structure, AlGaN is usedas a barrier layer and a clad layer in consideration of the confinementof carrier in a well layer.

However, these combinations have the following problems caused by highlydifferent optimal values of crystal growth conditions for AlGaN andInGaN. AlN has a high melting point as compared to GaN, and InN has alow melting point as compared to GaN. Therefore, the optimal growthtemperature is 1000° C. or below, preferably about 600–800° C., forInGaN, when GaN is 1000° C., and for AlGaN, the temperature is not lessthan that for GaN. When AlGaN is used for a barrier layer, the optimalcrystal growth conditions cannot be achieved unless the growthtemperatures of AlGaN barrier layer and InGaN well layer are changed,thereby problematically lowering the crystal quality. To change thegrowth temperature means discontinued growth, and in a well layer whichis a thin membrane having a thickness of about 3 nm, problems occur inthat the thickness changes due to an etching action during thediscontinuance in the growth, crystal defect occurs in the surface andthe like. Due to the presence of such a trade-off relationship, it isdifficult to obtain a high quality product by the combination of anAlGaN barrier layer and an InGaN well layer. Moreover, when AlGaN isused for the barrier layer, a distortion is imposed on the well layer,which problematically prevents high output. Thus, in the presentinvention, GaN was used as a material of the barrier layer in an attemptto reduce the above-mentioned trade-off problem, whereby the crystalquality was improved. With the aim of reducing the distortion, GaN wasused as an n-type clad layer, and the reduced distortion afforded highoutput. When GaN was used for a clad layer, insufficient confinement ofcarrier from InGaN having a composition capable of ultraviolet lightemission was worried, but it was found that the carrier (particularlyhole) could be confined.

In a different preferable embodiment for the output of ultravioletlight, the thickness of the barrier layer in the MQW structure islimited to 6 nm–30 nm, preferably 8 nm–30 nm, particularly preferably 9nm–15 nm. The thickness of the barrier layer in a conventionally MQWstructure is 3 nm–7 nm.

When the barrier layer is made as thick as this, the wave functions donot overlap at all, and it is more like SQW structures laminated inmulti layers, rather than a MQW structure, but sufficient high outputcan be achieved. When the barrier layer exceeds 30 nm, the hole injectedfrom the p-type layer is trapped by a dislocation defect to be anon-radiative center, which is present in the GaN barrier layer, and thelike before the hole reaches the well layer, and the emission efficiencyis unpreferably reduced.

The action and effect can be achieved that the well layer does noteasily suffer from the heat during growth of the layers above and damageby gas, because the barrier layer is thickened, thus reducing damages,and diffusion of dopant materials (Mg and the like) from the p-typelayer to the well layer is reduced, and further, the distortion imposedon the ell layer is also reduced.

EXAMPLES

Examples of actual preparation of GaN group LEDs having concavo-convexrefractive index interface according to the above-mentioned embodiments(I) and (II) are shown in the following.

Example 1

In this Example, concaves and convexes of a sapphire substrate wereburied by said facet growth method according to the above-mentionedembodiment (I) to give a concavo-convex refractive index interface and aGaN group LED was actually prepared as shown in FIG. 1( a).

A stripe pattern (width 2 μm, period 4 μm, stripe orientation: thelongitudinal direction of the stripe is <11-20> direction for GaN groupcrystal grown on the substrate) was formed on a C-plane sapphiresubstrate using a photoresist, etched with an RIE apparatus until thecross section became a 2 μm deep square, whereby a substrate having asurface having concaves and convexes of a stripe pattern was obtained asshown in FIG. 2( a). The aspect ratio then of the cross section of thestripe groove was 1.

After removing the photoresist, the substrate was set on an MOVPEapparatus and the temperature was raised to 1100° C. under a gasatmosphere (main component nitrogen) to conduct thermal cleaning. Thetemperature was lowered to 500° C. and trimethyl gallium (hereinafterTMG) was flown as a III group starting material, and ammonia as an Nstarting material, whereby a GaN low temperature buffer layer having athickness of 30 nm was grown.

Then the temperature was raised to 1000° C. and TMG and ammonia asstarting materials and silane as a dopant were flown, whereby an n-typeGaN layer (contact layer) was grown. The growth of the GaN layer wasthat burying the entirety, as shown in FIG. 2( b), occurring as aridge-like crystal having a facet plane and an angle cross section,which was generated from the upper surface of convex parts and thebottom surface of concave parts, without forming a cavity in the concavearts.

In the facet structure growth, when the C-plane of a GaN crystalcompletely disappeared and the top became a pointed convex, the growthconditions were changed to those rendering the growth in the lateraldirection predominant (elevating growth temperature and the like), and aGaN crystal was grown in a thickness of 5 μm from the top surface of thesapphire substrate. To obtain a burying layer having a flat top surface,membrane growth in 5 μm thickness was necessary.

Subsequently, an n-type AlGaN clad layer, an InGaN light-emitting layer(MQW structure), a p-type AlGaN clad layer and a p-type GaN contactlayer were successively formed to give an

substrate for an ultraviolet LED having an emission wavelength of 370nm. To expose an n-type contact layer, an etching processing, formationof an electrode and element separation were conducted to give an LEDelement.

The output of each LED chips (bare chip state, wavelength 370 nm,current flown 20 mA) obtained from the whole wafer was measured.

As Comparative Example 1, an ultraviolet LED chip was formed undersimilar conditions as above except that concaves and convexes in stripeswere not formed on a sapphire substrate (namely, an element structurewas formed on a flat sapphire substrate via a low temperature bufferlayer), and the output thereof was measured. The results of themeasurements are as described in the following.

Comparative Example 2

In this Comparative Example, a lateral growth method was applied using aconventionally known mask to achieve reduction of dislocation density ofthe GaN group crystal layer in the above-mentioned ComparativeExample 1. This Comparative Example 2 is a known constitution buryingthe mask at once with a single composition without-changing thecomposition during growth of a facet structure, and is vastly differentfrom the embodiment (particularly FIG. 5( b)) of the present invention(II) in that it does not have a concavo-convex refractive indexinterface from the growth of a facet structure.

A C-plane sapphire substrate of the same specification as in Example 1was set on an MOVPE apparatus and the temperature was raised to 1100° C.under a gas atmosphere (main component nitrogen) to conduct thermalcleaning. The temperature was lowered to 500° C. and TMG was flown as aIII group starting material, and ammonia as an N starting material,whereby a GaN low temperature buffer layer having a thickness of 30 nmwas grown.

Then the temperature was raised to 1000° C. and TMG and ammonia asstarting materials and silane as a dopant were flown, whereby an n-typeGaN layer (ca. 2 μm) was grown.

The substrate was taken out from the MOVPE apparatus, a stripe pattern(width 2 μm, period 4 μm, stripe-orientation: the longitudinal directionof the stripe is <11-20> direction for GaN crystal) was formed using aphotoresist, and a 100 nm thick SiO₂ was deposited with an electron-beamdeposition apparatus. The photoresist was removed by a method called aLift-Off Method to give a stripe-like SiO₂ mask.

The substrate was again set on the MOVPE apparatus and an n-type GaNcrystal contact layer was grown. The growth conditions were almost thesame as in Example 1, and the growth was continued after the growth fromthe exposed part of the GaN crystal (non-masked region) occurred as aridge-like crystal having a facet plane and an angle cross section untilit was flattened burying the whole. For burying, growth in about 5 μmthickness of GaN crystal in the C-axis direction was necessary.

Subsequently, an n-type AlGaN clad layer, an InGaN light-emitting layer(MQW structure), a p-type AlGaN clad layer and a p-type GaN contactlayer were successively formed to give an epitaxial substrate for anultraviolet LED having an emission wavelength of 370 nm. Furthermore, toexpose the n-type contact layer, an etching processing, formation of anelectrode and element separation were conducted to give an LED element.

The output of each LED chips (bare chip state, wavelength 370 nm,current flown 20 mA) obtained from the whole wafer was measured. Themeasurement results are as shown later.

Example 2

In this Example, a concavo-convex facet structure comprising an AlGaNcrystal was formed by said facet growth method according to theabove-mentioned embodiment (II), as shown in FIG. 1( b), which wasburied in GaN to give a concavo-convex refractive index interface,whereby a GaN group LED was actually prepared.

In completely the same manner as in Example 1, stripe patterned concavesand convexes were formed on a C-plane sapphire substrate, which was seton an MOVPE apparatus, and the temperature was raised to 1100° C. undera gas atmosphere (main component nitrogen) to conduct thermal cleaning.The temperature was lowered to 500° C. and TMG was flown as a III groupstarting material, and ammonia as an N starting material, whereby a GaNlow temperature buffer layer having a thickness of 30 nm was grown.

Then the temperature was raised to 1000° C. and TMG and ammonia asstarting materials, whereby a GaN layer was grown in about 100 nm. Thentrimethylaluminum (TMA) was added to the III group starting material andthe growth was continued, whereby AlGaN was grown. The growth of theAlGaN/GaN layer was that burying the entirety, as shown in FIG. 2( b),occurring as a ridge-like crystal having a facet plane and an anglecross section, which was generated from the upper surface of convexparts and the bottom surface of concave parts, without forming a cavityin the concave parts.

In the facet structure growth, when the C-plane of the AlGaN crystalcompletely disappeared and the top became a pointed convex, the growthconditions were changed to those for n-type GaN growth that rendered thegrowth in the lateral direction predominant, whereby an n-GaN crystal(contact layer) was grown in a thickness of 5 μm from the top surface ofthe sapphire substrate.

In completely the same manner as in Example 1, an n-type AlGaN cladlayer, an InGaN light-emitting layer (MQW structure), a p-type AlGaNclad layer and a p-type GaN contact layer were successively formed onthe n-type GaN contact layer to give an epitaxial substrate for anultraviolet LED having an emission wavelength of 370 nm. Furthermore, toexpose the n-type contact layer, an etching processing, formation of anelectrode and element separation were conducted to give an LED element.

The output of each LED chips (bare chip state, wavelength 370 nm,current flown 20 mA) obtained from the whole wafer was measured. Themeasurement results are as shown later.

Example 3

In this Example, a concavo-convex facet structure comprising a GaNcrystal was formed by said facet growth method according to theabove-mentioned embodiment (II), as shown in FIG. 4( c), which wascovered with 50 pairs of Bragg reflective layers having an AlGaN/GaNsuperlattice structure to give a concavo-convex multi-layer refractiveindex interface, whereby a GaN group LED was actually prepared.

In completely the same manner as in Example 1, stripe patterned concavesand convexes were formed on a C-plane sapphire substrate, which was seton an MOVPE apparatus, and the temperature was raised to 1100° C. undera gas atmosphere (main component nitrogen) to conduct thermal cleaning.The temperature was lowered to 500° C. and TMG was flown as a III groupstarting material, and ammonia as an N starting material, whereby a GaNlow temperature buffer layer having a thickness of 30 nm was grown.

Then the temperature was raised to 1000° C. and TMG and ammonia asstarting materials, whereby a GaN layer was grown as a ridge-likecrystal having a facet plane and an angle cross section, which wasgenerated from the upper surface of convex parts and the bottom surfaceof concave parts, as shown in FIG. 4( c).

In the facet structure growth, when the C-plane of the GaN crystalcompletely disappeared and the top became a pointed convex, 50 pairs ofAl_(0.2)Ga_(0.8)N (37 nm in C-axis direction)/GaN (34 nm in C-axisdirection) were grown, after which the growth conditions were changed tothose for n-type GaN growth that rendered the growth in the lateraldirection predominant, whereby an n-GaN crystal (contact layer) wasgrown in a thickness of 5 μm from the top surface of the sapphiresubstrate.

In completely the same manner as in Example 1, an n-type AlGaN cladlayer, an InGaN light-emitting layer (MQW structure), a p-type AlGaNclad layer and a p-type GaN contact layer were successively formed onthe n-type GaN contact layer to give an epitaxial substrate for anultraviolet LED having an emission wavelength of 370 nm. Furthermore, toexpose the n-type contact layer, an etching processing, formation of anelectrode and element separation were conducted to give an LED element.

The output of each LED chips (bare chip state, wavelength 370 nm,current flown 20 mA) obtained from the whole wafer was measured.

The measurement results (average value) of the above-mentioned Examples1–3 and Comparative Examples 1, 2 are as follows.

-   Example 1: 14 mW.-   Example 2: 14.5 mW.-   Example 3: 15 mW.-   Comparative Example 1: 6 mW.-   Comparative Example 2: 7 mW.

As is clear from the comparison of the above, by forming aconcavo-convex refractive index interface downward of the light-emittinglayer, a part of the light in the lateral direction, which disappearedwithin the element, could be taken out and the output of thelight-emitting element was found to have been improved.

Example 4

In this Example, a GaN group LED having a quantum well structure wasprepared as an embodiment wherein the layer between the light-emittinglayer and the crystal substrate comprised GaN alone.

A stripe pattern (width 2 μm, period 4 μm, stripe orientation: thelongitudinal direction of the stripe is <11-20> direction for GaN groupcrystal grown on the substrate) was formed on a C-plane sapphiresubstrate using a photoresist, etched with an RIE apparatus until thecross section became a 2 μm deep square, whereby a substrate having asurface having concaves and convexes of a stripe pattern was obtained.The aspect ratio then of the cross section of the stripe groove was 1.

After removing the photoresist, the substrate was set on an MOVPEapparatus and the temperature was raised to 1100° C. under a nitrogenatmosphere to conduct thermal etching. The temperature was lowered to500° C. and trimethyl gallium (hereinafter TMG) was flown as a III groupstarting material, and ammonia as an N starting material, whereby a GaNlow temperature buffer layer having a thickness of 30 nm was grown. TheGaN low temperature buffer layer was formed only on the top surface ofthe convex part and the bottom surface of the concave part.

Then the temperature was raised to 1000° C. and TMG and ammonia wereflown as starting materials, whereby an undoped GaN layer was grown forthe time corresponding to 2 μm on a flat substrate, and then the growthtemperature was raised to 1050° C. and the GaN layer was grown for thetime corresponding to 4 μm on a flat substrate. When growth is conductedunder such conditions, the growth of GaN layer then results in aridge-like crystal having a facet plane and an angle cross section,which was generated from the upper surface of convex parts and thebottom surface of concave parts, as shown in FIG. 2( b). By changing thegrowth temperature thereafter, a two-dimensional growth is promoted andthe GaN layer is flattened.

Subsequently, a three-cycle multiple quantum well layer comprising ann-type GaN contact layer (clad layer), a 3 nm-thick InGaN well layer(emission wavelength 380 nm, In composition was near nil, makingmeasurement difficult), and a 6 nm-thick GaN barrier layer, a 30nm-thick p-type AlGaN clad layer, a 50 nm-thick p-type GaN contact layerwere successively formed to give an ultraviolet LED wafer having anemission wavelength of 380 nm, which was followed by formation of anelectrode and element separation to give an LED element.

The output of each LED element (bare chip state), wavelength 380 nm,current flown 20 mA) obtained from the hole wafer was measured.

For comparison, an ultraviolet LED chip (Comparative Example 1) wasformed on a sapphire substrate free of concavo-convex processing underthe conditions similar to those in the above and the output wasmeasured.

In addition, an ultraviolet LED chip (Comparative Example 2) was formedon a typical substrate for ELO (GaN layer is once formed on a flatsapphire substrate, and a mask layer is formed) under the sameconditions as above, and the output was measured.

The results of the average value of dislocation density in the LED waferas measured by cathode luminescence, an average value of the output, andthe life in an accelerated test at 80° C., 20 mA (time for decreasing to80% of the initial output) are shown in Table 1.

TABLE 1 Dislocation density Output Life (dislocations/cm²) (mW) (hr)Example 8 × 10⁷ 10 1300 Comparative 1 × 10⁹ 3 800 Example 1 Comparative8 × 10⁷ 6 1300 Example 2

As is clear from Table 1, in this Example, the dislocation density isreduced and long life and high output could be achieved. As is clearfrom Comparative Example 2, while the dislocation density could bereduced by ELO method, which is one of the dislocation density reductionmethods, the output was lower as compared to this Example. This isconsidered to be attributable to difference in the crystallinity causedby the presence of a re-growth interface. In addition, since dislocationdensity on the substrate was generally high, both the output and thelife were poor as compared to this Example.

Example 5

In this Example, an ultraviolet LED chip was prepared In the same manneras in Example 4 except that an n-type Al_(0.1)Ga_(0.9)N clad layer wasformed between the n-type GaN contact layer and the InGaN well layer inExample 4, and the output thereof was measured.

As shown in the above-mentioned Table 1, the output of the element ofExample 4 was 10 mW, whereas the output of the element of this Examplewas 7 mW. This result has clarified that the element of this Exampleshows an increased output as compared to Comparative Examples 1, 2, andby removing the AlGaN layer from the InGaN well layer and the crystalsubstrate as in Example 4, the output could be further improved.

Example 6

In this Example, an experiment to examine the action and effect of alimitation relating to the thickness of the barrier layer of an MQWstructure was conducted.

A GaN group LED was prepared in the same manner as in theabove-mentioned Example 4 except that the thickness of each barrierlayer of the MQW structure in Example 4 was set to sample 1; 3 nm,sample 2; 6 nm, sample 3; 10 nm, sample 4; 15 nm and sample 5; 30 nm.These all belong to the light-emitting element of the present invention.

The output of the UV LED chip was measured under the same conditions asabove.

The average values of these measurement results are as follows.

-   Sample 1; 2 mW,-   Sample 2; 7 mW,-   Sample 3; 10 mW,-   Sample 4; 8 mW,-   Sample 5; 5 mW,

These samples were subjected to a photoluminescence measurement at a lowtemperature of 4K, the luminescence from Mg was observed around 3.2 eVin sample 1. This is considered to be the results of diffusion of Mgfrom a p-type layer due to a thin barrier layer.

As is clear from the above-mentioned results, the high output can beimproved at the barrier layer thickness of 6 nm–30 nm.

INDUSTRIAL APPLICABILITY

As mentioned above, by forming a concavo-convex refractive indexinterface downward of the light-emitting layer, the direction ofprogress of at least a part of the light in the lateral direction, whichis produced in a light-emitting layer, could be changed, and further,the amount of the light could be increased.

It has been made possible to provide a light-emitting element having anovel structure that permits entry of the light into a sapphiresubstrate, while suppressing the occurrence of a standing wave in thevertical direction, and particularly improves the light-extractionefficiency from the substrate side.

Moreover, by forming a crystal structure on a substrate processed tohave concaves and convexes, reduction of dislocation has been designed,by making the material of an n-type clad layer (also a barrier layer inquantum well structure) GaN, reduction of dislocation has been aimed,and by limiting the thickness of a barrier layer in a preferableembodiment of the MQW structure, the emission output of the element hasbeen increased, thereby achieving a long life.

This application is based on patent application Nos. 081447/2001 and080806/2001 filed in Japan, the contents of which are all herebyincorporated by reference.

1. A GaN group semiconductor light-emitting element having an elementstructure comprising a first crystal layer, which is a sapphiresubstrate processed to have concaves and convexes on its surface whereinthe concaves have a depth of 0.2 μm to 5 μm, a second crystal layerdirectly formed thereon or formed via a buffer layer, burying theconcaves and convexes, said second crystal layer being made from a GaNgroup semiconductor material having a different refractive index fromthe aforementioned crystal layer, wherein the second crystal layer isgrown from each of the concaves and convexes processed on the surface ofthe crystal substrate, while forming a facet structure comprising afacet plane which bends the dislocation line extending in the C-axisdirection from the substrate in the second crystal layer into thelateral direction, and continuously grown to make the surface flat, anda semiconductor crystal layer comprising a light-emitting layerlaminated thereon.
 2. The GaN group semiconductor light-emitting elementof claim 1, wherein the concaves and convexes processed on the surfaceof the crystal substrate have a stripe pattern, and the longitudinaldirection of the stripe is a <11-20> direction or a <1-100> direction ofa GaN group semiconductor grown while burying them.
 3. The GaN groupsemiconductor light-emitting element of claim 1, wherein the concavesand convexes have a cross sectional shape of a rectangular wave or atriangular wave.
 4. The GaN group semiconductor light-emitting elementof claim 1, wherein the difference between the refractive index of thefirst crystal layer and that of the second crystal layer at thewavelength of a light emitted from the light-emitting layer is not lessthan 0.05.
 5. The GaN group semiconductor light-emitting element ofclaim 1, wherein the light-emitting layer is made of an InGaN crystalhaving a composition capable of generating an ultraviolet light.
 6. TheGaN group semiconductor light-emitting element of claim 1, wherein thesecond crystal layer is grown via a low temperature buffer layer on theconcaves and convexes processed on the surface of the crystal substratewhile burying the concaves and convexes, the light-emitting layer is aquantum well structure comprising a well layer made of InGaN and abarrier layer made of GaN, and all the layers between the quantum wellstructure and the low temperature buffer layer are made of a GaNcrystal.
 7. The GaN group semiconductor light-emitting element of claim1, wherein the light-emitting layer is a quantum well structurecomprising a well layer made of InGaN and a barrier layer made of GaN.8. The GaN group semiconductor light-emitting element of claim 7,wherein the barrier layer has a thickness of 6 nm–30 nm.
 9. A productionmethod of a GaN group semiconductor light-emitting element comprising:processing a surface of a sapphire substrate as a first crystal layer tohave concaves and convexes on its surface wherein the concaves have adepth of 0.2 μm to 5 μm, forming a second crystal layer directly on thefirst crystal layer or via a buffer layer, burying the concaves andconvexes, and laminating at least one semiconductor crystal layercomprising a light-emitting layer on the second crystal layer, whereinthe second crystal layer comprises a GaN group semiconductor with adifferent refractive index from the first crystal layer, and wherein thesecond crystal layer is grown forming convexes from both the concavesurfaces and convex surfaces in the early stage of the growth to form afacet structure comprising a facet plane which bends the dislocationline extending in the C-axis direction from the substrate in the secondcrystal layer into the lateral direction, and is continuously grown tomake the surface flat.
 10. The production method of claim 9, wherein theconcaves and convexes processed on the surface of the crystal substratehave a stripe pattern, and the longitudinal direction of the stripe is a<11-20>direction or a <1-100> direction of a GaN group semiconductorgrown while burying them.
 11. The production method of claim 9, whereinthe concaves and convexes have a cross sectional shape of a rectangularwave or a triangular wave.
 12. The production method of claim 9, whereinthe difference between the refractive index of the first crystal layerand that of the second crystal layer at the wavelength of a lightemitted from the light-emitting layer is not less than 0.05.
 13. Theproduction method of claim 9, wherein the light-emitting layer is madeof an InGaN crystal having a composition capable of generating anultraviolet light.
 14. The production method of claim 9, wherein thelight-emitting layer is a quantum well structure comprising a well layermade of InGaN and a barrier layer made of GaN.
 15. The production methodof claim 9, wherein the second crystal layer is grown via a lowtemperature buffer layer on the concaves and convexes processed on thesurface of the crystal substrate while burying the concaves andconvexes, the light-emitting layer is a quantum well structurecomprising a well layer made of InGaN and a barrier layer made of GaN,and all the layers between the quantum well structure and the lowtemperature buffer layer are made of a GaN crystal.
 16. The productionmethod of claim 14, wherein the barrier layer has a thickness of 6 nm–30nm.